Tool wear has an important bearing on the performance of metal removal operations, where worn tools may result in scrapped workpieces due to unacceptable surface finish, out of tolerance dimensions, or damage caused by tool breakage. For these reasons, it has become common practice in machining operations to replace a cutting tool long before the end of its useful life, resulting in poor utilization of tools. Thus, there is the need for an effective method of measuring the amount of wear on a tool while cutting is in operation. At the present time, an acceptable tool wear sensing technology does not appear to exist.
Technologies have been and are being explored for monitoring cutting tool wear based on fundamental features and phenomena of wear and failure mechanisms that have been observed during cutting operations. Briefly, three distinct failure mechanisms have been identified:
1. Gross plastic deformation caused by excessively elevated temperatures.
2. Fatigue caused by excessively large cutting forces.
3. Gradual wear caused by the processes of adhesion, abrasion, electrochemical conversion and atomic diffusion.
The wear and failure mechanism that occurs in a specific situation depends on the cutting forces, temperature, and the tool and workpiece materials (e.g. composition, grain structure, surface composition). The variety of wear and failure mechanisms have resulted in various modes of testing and monitoring.
One such measurement involves the dimensional changes of the cutting tool or workpiece. This class of techniques includes mechanical gauging, profile tracers, weighing, ultrasonics, optical comparator methods and radiotracer methods. Except for radiotracer methods, all of these techniques are off-line measurements that frequently miss detection of the approach to failure. Also, radioactive methods are slow and perceived as unsafe.
The relationship between cutting forces and tool wear have been actively explored over the last twenty-five years, but a general correlation has not been established. For example, progressive flank wear produces increasing forces whereas progressive crater wear has the opposite effect. The observed forces also depend on material hardness, depth of cut and cutting speed. These techniques are hard to implement; requiring careful placement of strain gauges or dynomometers. Transducers frequently have to be incorporated into the original design of the machine.
Measurement of power consumption by the spindle or feed motor of a machining tool is easy to implement on new or existing machines. Such techniques have the potential of providing real-time optimization of metal removal rates, but serious disadvantages are inherent in these methods. Wear produces very small changes in power consumption which must be detected as a perturbation of a much larger signal (e.g. the overall power consumption of the motor). These methods are sensitive to non-wear related factors. Progressive wear of the tool increases power consumption but plastic deformation of the tool at high temperature decreases power consumption.
The bulk temperature of a tool can be measured by an embedded thermocouple or infrared technique. Infrared measurement has the disadvantage of requiring a very clean environment not found on a machine production floor, while embedded thermocouples require extensive redesign of a machine's spindle. The rise in the bulk temperature of the tool, caused by wear, is very small and the signal to noise ratio is poor.
Currently, vibration and sound analysis are active research areas, possibly because of the successful application of these methods to the study of rotating machinery and crack propagation in ceramics. The results of such analysis are very specific to a particular set-up and hard to generalize, and the techniques are difficult to implement, expensive and difficult to utilize. Therefore, a number of different techniques have been tried over the years the little success in the predictability of excess tool wear and/or failure. The present invention overcomes the deficiencies of previous techniques to provide an effective tool wear monitoring technique.